Particulates in diesel exhaust gas comprise principally of soot and uncombusted hydrocarbons. Depending on the engine used, the soot particle sizes are principally in the region of about 50-100 μm and are adsorbed on the surface of the filter until they are subsequently regenerated after appropriate coverage.
A functioning filter material which has become established for the specific use conditions in diesel filters is silicon carbide. Modern-day soot particulate filters are manufactured either by extrusion of SiC powder and subsequent recrystallization or directly from the individual silicon and carbon components. The silicization temperatures are in the range from 1500 to 2300° C. The particulate filter can be highly loaded and then burnt off thermally without any problems, the latter involving exhaust gas with a high oxygen content arriving at the filter at 600° C. in partial-load operation and the accumulated particles being degraded thermally.
To enhance the filter performance of the diesel particulate filters used, it is necessary to offer a maximum surface area on which the soot particles can be deposited. The extrusion process which has been used to date was able to create a relatively porous surface which, however, did not exhibit optimal adsorption properties.